1. Field of the Invention
The present invention relates generally to the fabrication of optical materials used in optical communication and integrated optical circuits using electron beam radiation and more specifically to a method for fabricating optical devices by inducing changes in the index of refraction in optical materials utilizing electron beam radiation.
2. Description of the Prior Art
Optical materials such as polymers and spin-on-glasses have found widespread use in optical communication and integrated optics applications. Typically, these optical materials are utilized to form waveguides and carry optical signals along a designated path. These optical waveguides are typically formed by utilizing materials of different refractive index. The inner waveguide material typically exhibits high optical transmission and has a higher optical index of refraction relative to the cladding or boundary material to maximize the internal reflection of the optical signal being transmitted and thus, minimize signal loss. To minimize signal loss, the ratio of the refracted indices of the inner and outer material are tailored to satisfy very specific specifications. The current state of the art of producing these waveguides and producing these materials of different index of refraction is to utilize two different materials, which are layered in an additive or subtractive process. The waveguides may also be fabricated by chemical vapor deposition of the different optical materials. In one prior art method, optical material having an intrinsic index of refraction is layered with another material, itself with an intrinsic (different) index of refraction, to create the layered waveguide.
In the prior art, the optical waveguide includes a core of material surrounded by a cladding of a dissimilar material than the core. The dissimilar material means that the material comprising the cladding and the material comprising the core are structurally and/or chemically distinct having been fabricated as physically different materials and brought together during the assembly process for the optical waveguide. Several methods are known in the art for waveguide formation, one being where an optical adhesive is used to bond the core to the cladding media. In those instances where an optical waveguide is on a chip, the fabrication process typically includes preparing a flat, optical surface on both the chosen core material and the chosen cladding substrate. Fused silica is typically used as a support substrate due to availability and low refractive index. In the case of very thin (adhesive) glue layers (<1 μm), the channel must be surrounded by a lower refractive index material for efficient waveguiding action. The refractive index of the selected optical core material dictates and limits the index range for the cladding or support regions. Other requirements of the surrounding medium are processing compatibility with the optical material, availability of the material, and adhesive bonding affinity.
In the case of thick glue layers, the refractive index of the glue provides the cladding index and influences the waveguide properties. In this situation, the support substrates may be selected for their processing qualities irrespective of the refractive index. The multiple layers of different materials create problems in fabrication as edge breakage and differential polishing rates between the glue and core/cladding materials must be taken into account as well as controlling appropriate glue thickness.
In all of these prior art techniques, the two dissimilar materials utilized to form the core and cladding layers must have the appropriate matched indices of refraction to achieve the desired optical coupling. These fabrication techniques are further complicated if optical adhesives are used which then must have the appropriate optical index for the device to work properly. For example, when distinctly different materials are used such as grating formation in waveguides fabricated using these techniques, complications arise due to the different exposure response of the two materials. This is especially bothersome in the case of narrow line width gratings. In the current state of the art, optical materials that can be processed which exhibit high transmission, low birefringence, and selected optical indices are very expensive and the range of indices is very limited, especially between compatible materials. Due to the explosion of optical networks and interconnects required by high-speed data transmission and the growth of the Internet, a need exists to fabricate material having a specific index of refraction and to perform multiple process steps in the same material. With such an ability, custom optical materials can be made from the same starting material or known materials can be tuned to work more advantageously with other materials.